Focused ion beams such as ion or electron beams are currently widely used for various types of integrated circuit analysis and manufacturing operations, notably characterization, identification, design and failure analysis, depassivation, vapor phase deposition, micro-machining, etc. These operations are performed using a particle beam production column designed to be focused onto the integrated circuit at the place intended for the desired intervention.
Such a column typically comprises a source of ions such as Ga+ produced from liquid metal which, after extraction, form an ion beam, which is then manipulated by a focusing device comprising a certain number of electrodes operating at determined potentials so as to form an electrostatic lens system adapted to focus the ion beam onto the integrated circuit. Each electrode of the focusing device, notably the output electrode, consists of a series of metallic electrodes having an aperture for passage of the particle beam. It should be noted here that the shape of the various electrodes as well as the aperture diameter plays a determining part in aberrations, notably spherical and chromatic aberration, of the particle focusing device.
One of the limits of applying focused ion beams is the impossibility of employing them to provide an in-depth image of a solid. Only surface images can be obtained. In the case of passivated and planarized integrated circuits, a surface image gives no information on the underlying layers and circuits, which has the disadvantage of making any intervention in the depth of the circuit extremely difficult such as, in particular, the cutting or breaking of buried metal tracks made necessary by design and failure analysis. To overcome this disadvantage, we employ an auxiliary light (photon) beam simultaneously and coaxially focused with the particle beam. In effect, using the light beam to obtain images in the thickness of the circuits, it is possible to visualize layers and tracks in depth and explore them, in real time, using the ion beam. It will now be understood that associating two types of beam, an ion and a photon beam, allows the operator to bring the ion beam exactly to the desired point on the object by means of the image supplied by the light beam.
Certain ion beam production columns also include an optical focusing device, a Cassegrain-Schwartzfeld (C-S) mirror objective lens for example, terminating at an outlet aperture placed close to the surface of a sample subjected to the ion beam.
French patent 2,437,695 discloses an emission ion lens associated with a C-S type mirror objective lens. In this system, the ionic lens part, the elements of which consist of two perforated electrodes and of the sample itself, is located between the object and the mirror objective lens. In this configuration, the apertures in the ion focusing device electrodes must simultaneously be sufficiently large to provide a geometrical expanse for the optical beam allowing sufficient sample illumination, and, relatively small so as not to deteriorate ion beam quality through excessive aberrations. The final diameter chosen for the outlet aperture is consequently a trade-off which is not satisfactory either for the optical beam extent or for ion beam focusing.
Secondly, the system disclosed in French patent 2,437,695 necessitates a very small (a few millimeters) working distance and the submitting of the sample to an electrical field. These two constraints are unacceptable in focused ion beam technology applied to integrated circuits: the danger of destroying the circuits by micro-electrostatic breakdown, impossibility of slanting the sample, difficulty in collecting secondary electrons, and the practical impossibility, through lack of space, of associating the system with a capillary tube for injecting pre-cursor gas which is an essential accessory in focused ion beam technology.